| Summary: | Indoor air quality, temperature, and relative humidity affect the thermal comfort and health of the inhabitants. Therefore, monitoring these parameters is essential to prevent indoor air pollution and to improve labor productivity. Most recently, the research conducted has used radio frequency wireless communications to monitor the indoor environment. However, long-term exposure to electromagnetic interference (EMI) causes a detrimental impact on the human body, especially on the elderly, patients, and infants, while it concurrently affects the operation of electronic devices. Thus, we propose a unique indoor environment monitoring system using an EMI-free bidirectional visible light communication technology. The sensing data from the smart sensor tag and the command data used to request the environmental sensing data are transferred bidirectionally between the base station and the smart sensor tag by being modulated into the light-emitting diode (LED) light beam. The proposed smart sensor tags are designed to measure five important parameters of indoor environments, such as CO<sub>2</sub> concentration, volatile organic compound level, O<sub>2</sub> concentration, temperature, and relative humidity, at multiple positions in real time. In addition, a proposed average-voltage tracking algorithm is adopted to allow the LED illumination to be used as a lighting system and as a long-range wireless communication system. As a result, the proposed system is demonstrated to be error-free when the distance between the base station and the smart sensor tags is 6 m. The measured live data are aggregated and visualized in a cloud platform by a graphical user interface. A web-based application and a mobile application are also developed to display the real-time data.
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